1. Photomultiplier (PMT) Tubes
Electron Cascade
Photon
Photocathode
Dynodes

2. Photocathode
Dynodes
Electron Focusing

3. A single photon will interact in the photocathode layer ~25% of the time (aka the “Quantum Efficiency”) to produce a single photoelectron, via the photoelectric effect.
Anode
This electron accelerates in the electric field and hits the first dynode, where it produces N (~2-4) electrons by secondary emission. These electrons accelerate to the next dynode, etc, etc, ending with the anode. At each stage, the gain is ~N, so if there are M acceleration stages, the overall gain in number of electrons is approximately M^N, with typical total gains being in the range 10^7 to 10^9.

4. A More Colorful Rendition of the Electron Cascade

5. A Better Detail of the HV Resistor Chain, and Signal Pickoff from the Anode

6. Quantum Efficiency vs. Photon Wavelength for Various Photocathode Materials

7. A Detail of Electron Focusing – Achieved by Shaping of Electric Field
Photon

8. A Scintillation Counter
A “scintillating material” (usually an inorganic crystal or doped plastic) emits light when its atoms are ionized or excited. This ionization can be caused by charged particles or by neutral particles that interact in the scintillator. In typical applications a single incoming particle will produce hundreds of scintillation photons, so that quantum inefficiency of the photocathode is not a problem.  Fortunately, the response of scintillation counters can be made proportional at each stage: production of scintillation light, production of electrons at the photocathode, and the gain of the dynode structure. This applies over a large dynamic range of light pulses, assuring linearity of the response over this range.